1. Field of the Invention
This invention relates to a chemical agent detector. More particularly, it relates to a device used for the detection of chemical agents, vapors and gases which utilizes a plurality of surface acoustic wave (SAW) based sensors in a microcontroller circuit for its sensing and detection functions.
2. Background of the Prior Art
Surface Acoustic Wave (or SAW) based sensors are known in the prior art. SAW sensor devices can be used in the detection of chemical agents, vapors and gases, aerosols and fluids. The detection of chemical agents which may be hazardous to the environment and to the health of living organisms has become very important. The safeguarding of workers in a factory or that of military personnel in hazardous environments has become increasingly important.
The use of SAW sensors for the detection of chemical agents are usually included in local oscillator circuits of detection devices. SAW sensors act as a key resonator in the local oscillator circuit. A polymer coating is deposited on the SAW sensor of which has a specific affinity to a specific gas or fluid to be detected. The oscillator circuit (or loop) is allowed to generate a constant frequency periodic wave. A frequency shift, and/or attenuation, in the oscillator output occurs when there is an introduction of the specific gas or fluid. This frequency shift is measured and used to determine that a specific gas or fluid is present in the environment. The frequency shift is a direct result of a decrease in velocity of the acoustic wave crossing over the SAW resonator. The decrease in velocity of the acoustic wave is a direct result of increased mass loading upon the sensor by molecules of the chemical agent, vapor, gas or fluid. In other words, changes to the propagation characteristics of the acoustic wave can be measured to indicate that the sensor has absorbed (or has been loaded with) a particular chemical agent.
U.S. Pat. No. 4,361,026 to Muller et al. discloses a device and method for sensing fluids wherein a single SAW sensor is employed for detecting a specific targeted fluid. A substrate made of silicon having a piezoelectric film deposited thereon is included on which surface acoustic waves can be propagated. Transmitting and receiving transducers are employed on a top surface of the silicon substrate and piezoelectric film for propagating and receiving the surface acoustic waves. A sensing member is also included along a bottom surface of the silicon substrate to interact with the surface acoustic waves; this causes a variation in the frequency of the wave for measurement by the receiving transducer in response to the presence of the specific targeted fluid. This invention is limited however in its measurement of targeted agents, wherein only one agent, and in particular a fluid, can be measured and detected. A multi-sensor array for the detection of a plurality of different agents is not disclosed or suggested. Further, this prior art invention does not contemplate how to process a high frequency signal that may be generated from the sensing member.
U.S. Pat. No. 4,895,017 to Pyke et al. discloses a device used for the detection of dilute chemical vapors that may be present in the ambient air. This detection device also utilizes SAW based sensors for implementing its detection function. The SAW sensor includes a substrate which is coated with a material selectively absorptive of a group of chemical substances of which the chemical substrate is a member. An electrical signal is produced which is indicative of a change in physical parameters associated with the coated surface in response to the chemical absorption that occurs. An analysis function is connected to the detection circuitry which determines a predicted time constant for diffusion of the detected chemical into the coated substrate and a predicted equilibrium concentration of the detected chemical into the coated substrate as a function of the produced electrical signal. Although this reference does teach the detection of a group of chemicals (that are similar in some way), it does not teach a device which permits the detection of a plurality of chemical agents that are dissimilar and not of the same group which can be detected by cycling a plurality of agent targeted SAW sensors through a multiplexer. Further there is no teaching or disclosure of how a high frequency signal received from the SAW sensor can be rapidly and efficiently processed and measured by a down conversion circuit and mixer. Yet further, there is no mention of utilizing a pressure/differential manifold for its air sampling function.
U.S. Pat. No. 5,325,704 to Mariani et al. discloses a SAW sensor array used for simultaneously detecting several chemical agents. The sensor array employs a bidirectional SAW transducer on a substrate. The substrate also includes several pairs of identical acoustic sensing and referencing channels, each located on opposite sides of the transducer in a mirror image fashion. Each channel is provided with a thin film for absorbing a chemical vapor or gas to be detected and are separated by an acoustic absorber. Also included in each channel is a metallic SAW grating reflector which receives and reflects surface acoustic waves through the film back to the transducer. The reference channels are shielded from ambient conditions while the sensing channels are exposed to the ambient conditions which may have a chemical agent present. An RF signal is applied to the transducer thereby causing an acoustic signal to be propagated through each channel and reflected back through the thin film to the transducer. These output signals are reflected back to the transducer and then detected and measured to see if certain propagation characteristics have changed which would be indicative of the presence of a targeted chemical agent. The specific characteristics include wave velocity and attenuation. This prior art reference improves upon this known type of SAW sensing device by miniaturizing the housing in which it is enclosed. However, this reference does not disclose, let alone teach or suggest, the use of a power cycling scheme with a plurality of SAW sensors to measure and detect a plurality of different chemical agents wherein a multiplexer is used to pass along the readings of the SAW sensors one at a time to a microprocessor for processing and alarm functioning. There is also no disclosure or suggestion of how to measure high frequency signals which may be generated by the SAW sensing device. Still further, there is no disclosure of mounting the SAW sensors on SAW driver boards directly upon a pressure/differential manifold.
Some prior art inventions have addressed power cycling schemes in their use of chemical agent detection devices that employ SAW multi-sensor arrays. This can be seen in U.S. Pat. No. 6,321,588 to Bowers et al. In this reference, a multi-sensor array is provided wherein a power multiplexor is employed to selectively provide power to each of the plurality of sensors at a specific time wherein only one sensor is turned on at any given time. This is done to conserve power and to alleviate cross-talk between the plurality of sensors. Since this reference is concerned with providing a compact detection device, power consumption is of great concern wherein a battery is employed to power the detection device. Since the plurality of sensors are exposed to a common ambient air environment, a need exists with this device to “clean” the sensors after they have been exposed to a chemical agent. The device of this reference addresses this problem by “scrubbing” the air and subsequently exposing the scrubbed air to the sensors. The scrubber utilizes a compound such as charcoal to clean the air sample. Nowhere in this reference does it disclose or teach the use of a novel pressure/differential manifold which can support a plurality of SAW sensors thereby providing a mechanism to expose each sensor to the air to be tested such that each sensor, when powered-on, is exposed to the air to be sampled at that given time and allowed to be analyzed, and then have that air sample evacuated by a pressure build up through the use of a valve in communication with an air intake port of the manifold. Further, this, nor any other prior art device, employs a phase tunable circuit for adjusting the frequencies of the SAW sensors to correct any minor, unwanted, yet inherent, differences that may be present on the polymer coated sensor due to manufacturer characteristics.
Clearly a need exists for an improved chemical agent detector which employs SAW sensing devices. Such a detector should utilize a plurality of SAW sensors that can be controlled by their own oscillator circuits. The sensors should be phase tunable to allow the sensors to be tuned to a more exact frequency than that which is set by the manufacturer. A power cycling scheme in combination with a signal multiplexor should also be incorporated to independently select each sensor such that only one sensor is “on” at any given time and its respective signal is processed at that same given time through a multiplexor. A down conversion element would ensure that all high frequency generated signals can be processed by a microprocessor. Finally, a novel approach to mounting the sensors on a pressure/differential manifold with a valve such that each sensor can be exposed to the air sample would alleviate a need to incorporate complex purging and scrubbing systems to clean the sensors after each air sample exposure.